1. Technical Field
The present disclosure relates to a biasing and driving circuit, based on a feedback voltage regulator, in particular a switching-mode power supply (SMPS), for an electric load.
2. Description of the Related Art
FIG. 1A shows a driving circuit for light-emitting diodes (LEDs), which is designated by the reference 1 and in particular includes a voltage regulator 5 of an SMPS type configured to operate as constant-current source and adapted to supply a string of LEDs 2 (illustrated in FIG. 1A is just one LED 2, coupled between output supply pins, VLED+ and VLED−). The SMPS device 5 is of a per se known type, for example described in the datasheet of the product “LED5000” manufactured by STMicroelectronics, entitled “LED5000—A monolithic step-down current source with dimming capability”, September, 2014.
The SMPS device 5 is operatively coupled to input supply terminals VIN+ and VIN−, present between which is a voltage VIN, for example generated by an electronic transformer (not illustrated).
The SMPS device 5 has, in a known way, a plurality of operating terminals, and in particular: a supply terminal 1a, adapted to receive an input voltage VIN, having a value, for example between 5.5 and 48 V; a reference terminal 1b, which forms a reference-voltage terminal; a feedback input terminal 1c, which is coupled to the sensing resistor 4 and constitutes the inverting input of an error amplifier internal to the SMPS device 5 (regulation terminal); a terminal 1d, which provides a power-supply connection for the internal analog circuitry; a terminal 1e, which forms, together with the reference terminal 1b and with the error amplifier, the output of a regulation loop internal to the SMPS device 5; and a terminal 1f that implements an output terminal for switching the SMPS device 5 and is coupled to the terminal 1d via a capacitor 8.
As illustrated in greater detail in FIG. 1B, the regulation loop internal to the SMPS device 5 includes the voltage-error amplifier 3, which implements a first stage of the regulation loop. In particular, the voltage-error amplifier 3 is a transconductance operational amplifier, the non-inverting input of which is connected to a voltage reference VREF internal to the SMPS device 5 (variation of which is typically between 194 and 206 mV; in particular, a typical value of 200 mV in closed loop is considered in what follows), whereas the inverting input terminal is connected to a sensing resistor 4. The inverting input terminal of the voltage-error amplifier 3 forms a feedback input terminal 1c of the SMPS device 5. The voltage-error amplifier 3 generates, on the terminal 1c, a control signal VCONTROL, which is supplied to the non-inverting input of a PWM comparator 7, which, in turn, drives, on the terminal 1f, the high-side (HS) switch of a DC-DC converter 9′. A current detector 9″ detects the current circulating in the high-side (HS) switch and supplies the value detected (transduced) to the inverting input of the PWM comparator.
The DC-DC converter 9′ generates at output a regulation signal VSW having a duty cycle such as to regulate the supply current (ILED) appropriately.
In other words, present between the terminal 1a and the terminal 1f is an SMPS converter, wherein the non-inverting input of the error amplifier acts on the terminal 1c, and the output of the amplifier acts on the terminal 1e. 
Thus, with reference to FIG. 1A, the SMPS regulator 5, the inductor 6 and the diode 11 form, for example, a DC-DC converter topology of a boost type.
The regulated current level supplied at output from the SMPS device 5 is thus set, or regulated, on the basis of the current that flows through the sensing resistor 4, across which, according to what has been said, there may be noted a voltage drop equal to the reference VREF of 200 mV. The resistance value RS of the sensing resistor 4 is consequently given by RS=(200 mV)/ILED, where ILED is the current that flows through the string of LEDs 2. In a case provided by way of example, where LLED=1A, we have RS=0.2 Ω.
Coupled to the terminal 1c of the SMPS device 5 a resistor 26 is further present, having a resistance R1 of approximately 10 kΩ. Optionally, it is possible to insert a Zener diode (not illustrated) in parallel to the resistor 26 so that the resistor 26 and the Zener diode implement a protection from overvoltages. The effect of the resistor 26 is negligible in so far as the current at input to the terminal 1c is substantially zero, or negligible (at the most a few tens of nanoamps).
Furthermore, coupled to the terminal 1e are a resistor 13 and a capacitor 15, connected together in series, which have the function of implementing a compensation network for the regulation loop. By way of example, the resistor 13 has a resistance of 22 kΩ and the capacitor 15 has a capacitance of 10 nF.
It is evident that the SMPS device 5 may include further input/output terminals, for implementing further functions, as required.
The input capacitor 10, coupled to respective supply inputs of the SMPS device 5, is configured to withstand the maximum operating input voltage and the maximum mean square value of the current. Capacitors adapted for this purpose, available for use for a wide range of currents, are, for example, electrolytic capacitors, ceramic capacitors, tantalum capacitors.
An output capacitor 12, coupled between the input VIN+ and the reference terminal 1b, has the function of filtering the current ripple of the diode 11, which, given a specific application and an output current, depends upon the value of inductance of the inductor 6. In general, if ΔIL is the current ripple of the inductor 6 and IL the average current that flows through the inductor, the value of inductance L is chosen in such a way that (ΔIL/IL)<0.5.
The driving circuit 1 may be coupled, as has been said, to an electronic transformer, which generates the input voltage VIN. Electronic transformers of a known type are typically based on a self-oscillating circuit and, to operate properly, require a load of a resistive type. In other words, the driving circuit 1 must be seen, by an electronic transformer coupled to the inputs VIN+ and VIN−, as a resistive load. However, it is known that an SMPS device, for example of the type illustrated in FIG. 1A and described with reference to that figure, in the absence of further arrangements, is seen as a load with negative impedance and thus is not optimized to be coupled to the output of an electronic transformer that requires a resistive load for its proper operation.
To overcome this drawback, it is known in the art to use a current control of the input signal. See, for example, Application Note 5372, “MR16 LED Driver Makes MR16 LED Lamps Compatible with Most Electronic Transformers” by Suresh Hariharan, Mar. 27, 2013, Maxim Integrated Products. A similar solution is discussed in the datasheet of the product MAX16840, manufactured by Maxim Integrated Products, Inc., “LED Driver with Integrated MOSFET for MR16 and Other 12V AC Input Lamps”.
In this technical solution, represented schematically in FIG. 2, the voltage on the sensing resistor 4 is regulated at each switching cycle, exploiting a reference circuit 18 external to the SMPS device 5, adapted to supply a voltage signal VREFI to a further input terminal 1g of the SMPS device 5, for the purpose of setting the input current level by appropriately controlling the voltage on the terminal 1c. In other words, when the voltage VREFI present on the terminal 1g drops below a certain threshold value, the input current (voltage on the resistor 4) is regulated proportionally to the value assumed by the voltage VREFI on the terminal 1g. Instead, when the voltage VREFI present on the terminal 1g exceeds the threshold value, then the input current (voltage on the resistor 4) is set at a predefined fixed value. The voltage on the sensing resistor 4 is thus regulated as a function of the voltage VREFI received at input on the terminal 1g, which is in turn a function of the input voltage VIN. This type of modulation of the voltage on the terminal 1c enables simulation of a resistive load, seen by an electronic transformer coupled to the input of the driving circuit 1 of FIG. 2. However, this implementation requires a terminal of the device 5 (terminal 1g) explicitly dedicated to this purpose, a circuitry internal to the device 5 adapted for regulating the voltage on the terminal 1c as a function of the reference on the terminal 1g, as well as, at the same time, an external circuit for generating the reference signal to be supplied to the terminal 1g. In other words, this solution is not applicable to any generic SMPS device; the latter, instead, must be purposely built.
Other known solutions require provision of dedicated dual-stage converters, with consequent implementation of double inductive components, which increase the costs and size.
There is thus a need to provide a driving circuit for a voltage regulator, for example of an SMPS type, that is adapted to emulate a resistive load when seen from the input terminals VIN+ and VIN−, is such as to increase the power factor, with lower production costs and reduced occupation of space, and is able to operate with any generic voltage regulator.